How to take your heart rate on iPhone, no additional monitor required!

Even though the iPhone doesn't have a built-in heart rate sensor, you can still find a third-party app for that!
Apple has put a much larger emphasis on health and fitness in iOS 8 than in any other version of iOS *yet the iPhone still doesn't offer a heart rate monitor in the Health app. That being said, there are third-party apps in the App Store that can pick up the slack, and don't even require an external sensor of any kind!